I am confused with the usage of The Present Perfect in a North American textbook vs British English.

Please read this dialogue:
A: You have taken part in some volunteer activities during the summer holidays
B: Yes, I have been a volunteer in a children's summer camp.
A: Did you clean the children's rooms?
B: Yes, I did......

IMO the highlighted verbs should be in the Past Simple Tense. Reason: Past Simple is used with a finished time period. Moreover all references to the 'summer holidays' later in the text are in the Past Simple.

In another piece of text the author references the same time period using The Past Simple:

A: Hi, B. Did you have a good summer holiday?

Can anyone explain according to which rule in North American English the finished time period 'summer holiday' should be referenced by The Present Perfect? How can I explain the difference between Past Simple and Present Perfect to my students, using a North American textbook?

References: Michael Swan, Practical English Usage, 3rd edition, OUP, 2009